BB,
some dr find that D5 embryo is better becos it sorts of prove that they are stronger than D3, therefore have better chances of surviving when put back to our body, I have 5 frozen embryos, 2 D7 and 3 D5.
Our body condition and lab condition is different, embryos are suppose to grow inside our body condition so some advised that it is better to put it in asap, when it is D3 we already can tell which are the stronger ones, like you mentioned, some after D3 might dies off after D4 or D5 or become bad, so they need to replace it to your body asap, but if you have lot of good embryos, they can afford to wait till D5 to put it in. But then again, renee mentioned to me before that sometimes when the embryos has been in the lab condition for longer period, it might not get used to body condition anymore so will be difficult to get implant. Whether it is D3, D5, I am sure the dr and embryologist knows best when to replace it in for us, so just trust them to know lor, in fact, having good grade embryos does not mean you will get pregnant, similarly some gets pregnant with average embryos, heng sway issue. Relax and and keep your mood constant at all time, will help implantation.
BTW you have to tell Dr Xia Rong when you doing your FET, she will need to change the medicine and do different accup. on you when you are going to start.